In a conventional nuclear reactor plant, a nuclear reactor such as a boiling water reactor (BWR) is submerged in reactor water within a pressure vessel, and the pressure vessel is disposed inside a containment building. During operation, the reactor core boils the reactor water to generate reactor steam which is suitably channeled to a steam turbine, for example, for generating electrical power. The pressure vessel is suitably sized and configured for containing the relatively high pressures of the reactor steam which may be about 70 kg/cm.sup.2 for example. The containment building, in turn, is sized and configured for also containing such relatively high reactor steam pressures in the event of failure of the pressure vessel or the reactor steam lines therefrom. The containment building also is effective as a radioactive shield for containing radioactivity therein. The building is typically made of thick concrete and is metal lined.
In one type of failure mode of the reactor plant, the reactor may become isolated from its normal cooling and water makeup systems, and a conventional isolation condenser is provided to cool the reactor in such event. In another failure mode, a pressure boundary failure such as the failure of one of the reactor steam pipes within the containment building may release hot steam under reactor pressure inside the containment building, and the isolation condenser may also be used to cool that released steam. In either occurrence, as the reactor core is shut down, decay heat is generated which in turn continues to generate reactor steam which must be quenched and cooled to prevent unacceptable temperature and pressure rises within the pressure vessel and/or the containment building.
The isolation condenser is typically a conventional heat exchanger having a plurality of tubes therein which are disposed within an isolation pool of water outside the containment building, and the reactor steam from the pressure vessel or from within the containment building is suitably channeled to the isolation condenser and between its tubes for cooling the steam and transferring the heat thereof to the isolation pool water. The reactor steam is condensed on the tubes and is conventionally drained back to the pressure vessel and reused to carry more heat away from the reactor core.
In order for the isolation condenser to be effective for maximizing heat transfer from the reactor steam to the pool water, the tubes must be relatively thin and single walled, but, they must be also strong enough to contain the relatively high pressure of the reactor steam being channeled therethrough. Since the reactor steam is channeled through the containment building and through the condenser tubes disposed outside thereof, the tubes themselves provide only a single barrier against release of the reactor steam, which is radioactive. If one or more of the condenser tubes fails during operation, the reactor steam will leak into the isolation pool and be released through a conventional vent to the atmosphere, which therefore would release radiation to the atmosphere outside the containment building.
In order to reduce the risk of radioactive steam release from the condenser in the event of a failure thereof, conventional isolation valves are provided both in the conduits leading from the pressure vessel or containment building to the isolation condenser and in the conduits returning the condensed steam back to the pressure vessel. The isolation valves are normally closed valves which must be energized to open during operation so that, upon any failure of the isolation condenser which might release steam therefrom, the fail-safe condition will allow the valves to close upon interruption of power thereto which will stop the flow of reactor steam to the isolation condenser and, therefore, prevent any further release of radiation to the atmosphere.
Accordingly, this exemplary conventional isolation condenser system provides a single barrier against release of radioactive steam and is an active system in part since power must be provided to the isolation valves to keep them open during operation while allowing the fail-safe closure thereof in the event of interruption of power thereto to reduce the risk of inadvertent release of radiation in the event of isolation condenser tube failure.